1. Field of the Invention
The present invention relates generally to semiconductive ink compositions, and more particularly, to compositions and methods for printing thin films of IB-IIIA-VIA compound semiconductive material useful in manufacturing thin film solar cells, and particularly monolithically linked or mechanically stacked tandem solar cells with improved efficiency and lowered cost.
2. Description of the Related Art
Copper indium diselenide and its derivatives of gallium and sulfur substituted compounds can be generalized as CuInxGa1-xSe2S2-y (for 0≦x≦1, 0≦y≦2), and can often be called CIS, CIGSe or CIGSeS. They are IB-IIIA-VIA semiconductor material and are widely used in thin film solar cells, due to their favorable electrical and optical properties, stability, and energy conversion efficiency. The chalcopyrite material is a tetrahedrally-bonded semiconductor, with a bandgap varying continuously with x from about 1.0 eV for copper indium diselenide to about 1.7 eV for copper gallium diselenide, and with a bandgap varying continuously with y from about 1.45 eV for copper indium disulfide CuInS2 to about 2.38 eV for copper gallium disulfide CuGaS2. Since Wagner made the first single crystal CuInSe2 solar cell with 12% efficiency in 1973, much progress has been achieved. So far, the highest solar cell efficiency with the most reliable yield was shown by physical vapor evaporation (PVE) process (with the highest cell energy conversion efficiency of 19.9% (See Ingrid Repins, Miguel A. Contreras, Brian Egaas, Clay DeHart, John Scharf, Craig L. Perkins, Bobby To, Rommel Noufi, 19.9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor, Progress in Photovoltaics: Research and Applications, 16, 235 (2008)). However, PVE process is difficult for large area commercial scale PV production due to its point-evaporation nature, and due to the difficulty for uniform composition control. To date, the CIS modules produced are still too expensive to compete with polycrystalline Si based solar modules.
To overcome these hurdles and to achieve a better control of the Cu/(In+Ga) ratio throughout the film, attempts have been made to fix this ratio in a material before the deposition process, and then transfer this fixed composition into the thin film formed using the material. One initial attempt was a screen printing technique that use a paste of milled fine powder of Cu, In and Se in the compositional ratio of 1:1:2 to form a preliminary Cu—In—Se film on a borosilicate glass substrate, followed heating to 700° C. in a nitrogen atmosphere to form a semiconductor compound film of CuInSe2 (T. Arita et al, 20th IEEE PV Specialists conference, 1988, page 1650). Due to the non-uniformity of composition caused by the large metal particle size (up to 2 μm), and the high sintering temperature, which causes indium loss and deforms the soda-lime glass substrate, PV performance was reported to be low, with efficiencies of only about 1%. Also, In(OH)3 or In2O3 may be formed in the sintered films, as indium powder easily oxidizes at high temperatures in the presence of trace amounts of oxygen.
Another attempt was to prepare chalcogenide nanoparticles by reacting iodides of copper and indium with sodium selenide in an organic solvent bath system such as a mixture of pyridine and methanol, as described in Schultz et al., U.S. Pat. No. 6,126,740. Nanoparticles of CuInGaSe2 in the range of 10-30 nm can be obtained, and their suspension in mixture solvent of pyridine/methanol was sprayed directly onto a molybdenum coated soda-lime glass substrate heated to 144° C. With this technology, a film with fixed ratios of the four elements is readily achieved. However, the CIGS nanoparticles are largely amorphous and the formed film is not desirable for high performance photovoltaic cell. The amorphous condition of the particles may be due to the fast reaction between the iodides and sodium selenide in the pyridine-methanol medium. Besides, the large quantity of sodium iodide byproduct in the film may interfere the formation of crystalline particles.
Recently, Kapur et al. disclosed an oxide-based method of making IB-IIIA-VIA semiconductor compounds (U.S. Pat. No. 6,127,202) in which an ink of oxide-containing particles including Group IB and IIIA elements is formed by pyrolyzing metal nitrates or sulfates of IB and IIIA elements (such as copper and indium) into fine oxide particles. A non-vacuum solution coating method can produce a thin film of Cu2In2O5 from these particles, and the film can be transformed to copper indium diselenide (CIS) by treatment in hydrogen, hydrogen selenide, or both at an elevated temperature (425-550° C.). Similarly, Cu2In2-xGaxO5 can be formed and transformed into a CuIn(Ga)Se2 film as disclosed by Eberspacher et al (U.S. Pat. No. 6,268,014). Both techniques utilize the non-volatility of the oxides of IB and IIIA metals, and chemically reduce the oxides while adding selenium to form an IB-IIIA-VIA thin film. Although precise control of the IB/IIIA elemental ratio is readily achieved by this method, the extra reduction and “selenization” process of the oxides are both complicated and costy, which limits the benefit of this non-vacuum process over co-evaporation process. Besides, the difficulty to remove completely the trace oxide of gallium and indium also limits the performance of thus formed solar cells. (X. Charles Li et al, Proc. of SPIE Vol. 7047, 12(2008), Brian Sater et al, U.S. Pat. No. 7,306,823).
To overcome the non-uniformity and the complex reduction/selenization process associated with IB-IIIA oxides, a most recent disclosure utilizes non-oxide nanoparticles of IB-IIIA-VIA that are coated with one or more layers of indium metal (Brian M. Sager, et al, U.S. Pat. No. 7,306,823). Dense precursor films of IB-IIIA-VIA are expected to form upon heating the coated nanoparticles. With this precursor process, the second selenization process is still necessary to drive the metals and the chalcogen to react and form semiconductive chalcopyrite. Besides, due to the different density with various metals of Cu, In, and various compounds, the liquid suspension or the ink is not well formulated suitable for high throughput printing purpose.
With an aim to directly deposit CIGS thin films, nanoparticle CISe solutions have been recently used to form solar cells (Qijie Guo et al, NanoLetters, 8(9), 2982(2008)). This direct printing process has the true nature of high throughput low cost potential. However, the thin film solar cell fabricated by these printing processes still have sacrificed performance compared with their counterparts fabricated by metal oxides inks or fabricated by reactive co-evaporation, largely due to the ink materials used not being formulated or designed for printing applications for solar cells. High efficient CIGS solar cell not only requires a precision control of chemical composition, but also requires large crystal grain size (>1 μm) thin film with excellent composition uniformity; this large crystal grain size requirement contradicts to the formation of nanocrystal CIGS ink wherein smaller size favors more soluble and more stable nanocrystal CIGS ink. It will be appreciated that there is a need in the art for the preparation and formulation of CIGS inks designed for direct printing of CIGS based solar cells.